Formula based on user input

Is there a formula or vba that will take user input from a couple of cells and use a particular formula in another cell chosen based on the user input?

For example, if user enter Week # = 1, # of Days = 100, then Projected Month End needs to calculate 100/95. But if user enters Week # = 2, # of Days = 90, then the Projected Month End needs to calculate 90/80.

It's hard to write down everything I'm trying to do with the spreadsheet all in one question. So I was just going to post it in parts in seperate questions.

Basically, I want a dialog box type of app that a user can quickly view projected end of month sales. The projection formula is easy.
If it is the beginning of the month (BOM) then the current days are divided by .96 to predict the end of month days. One week out, projected = current/.93.
2W = current # of days/.92
3W = curret/.88
4W = current/.84
5W = current/.80
6W = current/.76
7W = current/.71
8W = current/.65
9W = current/.60

I know I can use nested IFs in the Projected Month End cell. I was thinking there might be a more elegant way of programming it using VBA. I would hate the IFs formula to get accidentally overwritten.

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